There is a "pink blob" type character that got very famous in Japan before all of those, and has the properties of
a.) shapeshifting
b.) being food-based
c.) being pink
The character comes not from Japan, but from France.
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In 1970, the wife-and-husband team of Annette Tison and Talus Taylor (she was French, he was American, they met in Paris) had heard a small child asking for a treat saying "babababababa". With this sound inspiration struck and they used cotton candy as the basis to to draw Barbapapa (the name means candy floss, that is, cotton candy).
The first children's book, Barbapapa, introduces the title character who has grown in a garden. The family that owns the garden takes Barbapapa to a zoo, where he escapes and rescues people from a fire by shapeshifting into a staircase and becomes a town hero.
He can take any form. He is very nice, everybody likes him.
Not far from the village where his friends Cindy and Frank live, he has built a house like him for his family.
With a few shape shifting and a brilliant imagination, he smoothly overcomes the most difficult situations!
Later stories introduce a female of the same species (Barbamama) and the two make a seven-children family (Barbabravo, Barbabright, Barbazoo, Barbabeau, Barbalala, Barbabelle, and Barbalib). They moved into a house inspired by the work of avant-garde architect Antti Lovag who designed "bubble houses" where everything is shaped around circles.
The books are framed around environmental issues, and in one of the most famous books, Barbapapa's Ark, the Barbapapa family turns into a rocket ship so the animals can all board leave the too-polluted planet; the humans, realizing the error of their ways, clean the planet so the animals can return.
The books were quickly adapted as a joint Dutch-Japanese television production which aired in multiple countries including -- pertinent for our discussion -- Japan in the mid-1970s. Again: it is still famous in Japan and a special café dedicated to the character and its universe opened in 2015.
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Now, of the main characters mentioned -- let's say Kirby, Jigglypuff, and Majin Buu -- none of the authors have gone on the record of thinking about Barbapapa, even though it was quite strong in the cultural consciousness (it'd be like not thinking about Speed Racer; it's possible, but it at least bears some scrutiny).
Masahiro Sakurai, for example, worked on the first Kirby game for Game Boy, and used simple "placeholder art" which ended up becoming the final art. There were multiple colors theorized for Kirby (like white on the North American box for the game) but Masahiro Sakurai always thought the character should be pink ... for a shapeshifting blob character that eats in order to change shape. It could have been an entirely independent decision, of course, but the possibility must still be raised.
There aren't any good interviews I've seen regarding Jigglypuff, but the original Japanese name (Purin) means a custard or pudding; that is, just like Barbapapa, it was designed whilst thinking of food in pink blob-like manner. (Jigglypuff doesn't shapeshift, though. That'd be Ditto, who in its natural state is a purple or pink blob. Also, to mention Clefairy and Chansey, Jigglypuff -- and Clefairy -- were changed in a later generation to be fairy-type to match. Incidentally, another word for "cotton candy" or "candy floss" is "fairy floss".)
Majin Boo, designed by Akira Toriyama, was an attempt in his Dragon Ball series to make a very different villian from any previous one; he came up with a shapeshifting pink genie that has existed since the beginning of time. One of his forms is a "fat form" and he generally likes to eat candy:
He doesn’t particularly need energy. It’s just that he likes candy.
None of the creators have gone on the record as being inspired by the children's show, so it would be irresponsible to say with certainty there is a link, but it is possible to be inspired by something entering the cultural air -- pink shapeshifting food creatures -- without ever being conscious of the source.
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You can watch an English-dubbed episode of the Barbapapa show (a more recent one) here.
The pink blob motif isn't entirely new in Japanese Culture. For example:
Lantern Plants Imitate Children at Play, Utagawa Hiroshige, 1842
Jinmenju - The Human Face Tree
This yokai, or spirit, which was the basis of the pokemon Exeggutor, is likely derived from the Waq Waq Tree of Islamic tradition: Waq Waq Tree and is portrayed in multiple prints as faces on fruits.
A regional story of animated persimmons.
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However lets get to the heart of the matter with Kirby, the shapeshifter, this too might have roots in traditional Japanese folklore.
Tanuki (Good source for wood block prints of the Edo period)
Perhaps the most famous of Japanese spirits in Western Culture are the Tanuki. Infamous shapeshifters well endowed with testicles and scrotums that they expand and shrink and stretch to serve a huge variety of purposes from fishing nets to umbrellas to weapons.
Hopefully this is a helpful. I am a novice enthusiast, but hadn't seen anyone answer from this angle, so hopefully proper scholars on Japanese folklore can weigh in. If you want to read more on the subject here are several western books i've read that were pretty fantastic, though certainly western in perspective. If anyone could suggest other books i'm all ears, I adore this topic!
"Pandemonium and Parade: Japanese Monsters and the Culture of Yokai" By Michael Dylan Foster, 1965
"The Book of Yokai: Mysterious Creatures of Japanese Folklore" by Michael Dylan Foster, 1965
"The Night Parade of One Hundred Demons: A Field Guide to Japanese Yokai" by Matthew Meyers, 2012
"The Hour of Meeting Evil Spirits: An Encyclopedia of Mononoke and Magic" by Matthew Meyers, 2015
"The Book of Hakutaku: A Bestiary of Japanese Monsters" by Matthew Meyers, 2018
